Hello,

I'd start by monitoring disk I/O of the drive that holds TEMPDB files now to see if you get heavy usage to your TEMPDB files. We have some high usage OLTP systems and there's often surprisingly little going on the TEMPDB disks, especially from 2005 and onwards (thanks to improved caching and other improvements).

Personally I tend to go with dedicated disk and LUN for TEMPDB, then place both data- and log files on it. In SQL 2005 and newer there's some improvements made to TEMPDB that reduce the logging overhead and I haven't had any problems with data- and log files sharing the same disk.